1. Field of the Invention
This invention relates to electronic circuits which may be used in computer systems and/or integrated circuits and, more particularly, to a voltage-dependent impedance selector for non-linearity compensation in a computer or other electronic system and/or an integrated circuit (IC).
2. Description of the Related Art
Voltage dividers are frequently used in electronic circuit design. Generally, a voltage divider outputs a voltage that is a fraction of an input voltage. FIG. 1 illustrates a simple voltage divider made from two resistors, R1 and R2, placed in series across an input voltage V. The output voltage is taken across R2 and equals V*R2/(R1+R2).
In electronic circuit design, it may be desirable to create a circuit that is capable of operating at several different operating voltages. For example, a circuit may be designed to operate with several different versions of a component. Each version of the component may operate at a different voltage, so the circuit may need to be able to operate at each of the possible operating voltages in order to be compatible with the different versions of the component. However, design of such a circuit may be frustrated if non-linearities arise when the circuit""s operating voltage changes.
One possible non-linearity may arise when a circuit uses a reference voltage. A digital component may use a reference voltage to distinguish between logical states of a signal. For example, if a particular signal""s magnitude is higher than the magnitude of the reference voltage, the signal may be interpreted to be high. This may correspond to a logical 1 for an active high signal or to a logical 0 for an active low signal. Conversely, if the signal""s magnitude is lower than that of the reference voltage, the signal may be interpreted to be low. Often, a voltage divider across another voltage generates the reference voltage. As a result, the reference voltage equals a set percentage of that other voltage. In some circuits, however, certain components may not perform optimally at some or all of the possible operating voltages if the reference voltage is always set at the same percentage of another voltage. In these circuits, the optimal reference voltage percentage may vary according to which operating voltage is used. Thus, the optimal value of the reference voltage percentage may not vary linearly with the operating voltage of a circuit.
Another non-linearity may arise when designing an electronic circuit that includes impedance-matching circuitry. In circuits designed to be used in systems where signals are transmitted at very high speeds, impedance-matching circuitry may be used to reduce signal distortion caused by unmatched impedances along the transmission path of a signal. However, impedance-matching circuitry may be designed to operate at one operating voltage. As the operating voltage is changed, non-linearities may arise in the impedance-matching circuitry. These non-linearities may cause the impedance-matching circuitry to not perform optimally, creating a mismatch between impedances in the transmission path and resulting in unwanted signal reflections or other transmission problems.
Various embodiments of a voltage-dependent impedance selector and methods of selecting an impedance based on an operating voltage are disclosed. In one embodiment, a voltage-dependent impedance selector may include a plurality of discrete impedances, a selection stage, and an output stage. The selection stage may be configured to dynamically select one of the discrete impedances depending on the operating voltage. When the operating voltage equals the first voltage, the selection stage may dynamically select the first impedance, and when the operating voltage equals the second voltage, the selection stage may dynamically select the second impedance. In one embodiment, the selection stage may include a comparator that compares the operating voltage to a voltage threshold. The selection stage may then indicate that the operating voltage equals a first voltage (or is within a first voltage range) if the operating voltage is greater than the voltage threshold and that the operating voltage equals a second voltage (or is within a second voltage range) if the operating voltage is less than the voltage threshold. The selection stage may couple an impedance to the output stage, which has its own impedance, to form a selected voltage divider. The impedance that is coupled to the output stage includes the impedance of the selected impedance and, in some embodiments, may also include the impedance of the selection stage. The selected voltage divider may divide an input voltage to generate an output voltage. The input voltage may be the operating voltage in one embodiment.
In some embodiments, the selected voltage divider may provide the output voltage as a reference voltage to another component so that the component may use the reference voltage to distinguish between logical states of a signal. The selected voltage divider may be configured to generate a reference voltage equal to a first percentage of the input voltage when the first impedance is selected and to generate a reference voltage equal to a second percentage of the input voltage when the second impedance is selected.
In other embodiments, the output stage may include an impedance matching compensation circuit, and the selected impedance may be a reference resistor. The impedance matching compensation circuit may match the impedance of an output driver to the impedance of the selected impedance.
In one embodiment of the voltage-dependent impedance selector, the selection stage may also include two transistors. One transistor may be coupled between the first impedance and the output stage and configured to turn on when the operating voltage is greater than the voltage threshold. The second transistor may be coupled between the second impedance and the output stage and configured to turn on when the operating voltage is less than the voltage threshold.
One embodiment may include a method for selecting an impedance. The method may include receiving an input voltage, determining the magnitude of an operating voltage, selecting a impedance from a plurality of discrete impedances based on the magnitude of the operating voltage, and dividing the input voltage across the selected impedance and an output stage to generate an output voltage. A first impedance may be selected if the operating voltage equals a first voltage (or exceeds a threshold) and a second impedance may be selected if the operating voltage equals a second voltage (or is less than the threshold).
In one embodiment, the method may also include comparing the output voltage to a signal (or otherwise using the output voltage) to determine the logical stage of the signal. In another embodiment, the method may include matching an output impedance of an output driver to the selected impedance.